1. Field of the Invention
This invention relates to a double beam scan type optical apparatus. Such apparatus are used in laser printers and the like.
2. Description of the Related Art
In a laser printer in which the laser beam is deflected in a scan mode, in order to increase the scanning speed or scanning accuracy it is necessary to increase the number of times that the scanning beam strikes the scanning surface. In order to meet this requirement, a method exists in which a plurality of light sources are provided, and the output light beams of these light sources are simultaneously deflected in a scan mode. That is a beam position control method by servo control using a light position detector for maintaining the accurate mutual positions of a plurality of light sources.
FIGS. 9 through 12 (prior art) show one example of a conventional laser beam control method using two light sources (cf. the publication "Ohyou Hikari Erekutoronikusu (Opto-electronics Applications) Handbook", April 1989). As shown in FIG. 9 (prior art), the output light beams 11a and 11b of two light sources 1a and 1b are collimated by coupling lenses 2a and 2b so that they are applied to a light splitter 4 by way of movable reflectors 3a and 3b, respectively. As a result, the two light beams are split in such a manner that they advance towards a light deflector 5 and a light position detector 10.
The light beams advancing towards the light deflector 5 are focused on a drum surface 7 by an F.theta. lens 6 or the like, so that two scanning lines 18a and 18b are formed as the light deflector 5 rotates. The light beams 11a and 11b forming the two scanning lines 18a and 18b are moved across a light scan detector 8 with a distance P therebetween in a main scanning direction, and the light scan detector 8 outputs light beam (11a and 11b) passage signals 19. A control device 16 receives the light beam passage signals 19 as reference signals, and applies printing signals, as modulating signals 12a and 12b, to the light sources 1a and 1b to modulate the light sources 1a and 1b. As a result, printing information is applied to the drum surface 7.
On the other hand, the light beams advancing towards the light position detector 10 are focused on the surface of the light position detector 10 by a focusing lens 9. The light position detector 10 is a position detecting sensor for maintaining the relative positions of the two light beams 11a and 11b constant. The drum surface 7 and the light position detector 10 are in the focusing planes of the lens systems. Therefore, the beam diameters and the beam distances on the drum surface 7 and those on the light position detector 10 are in proportion to each other. Hence, the distance (d) between the auxiliary scanning directions of the scanning lines 18a and 18b on the drum surface 7 can be maintained constant by maintaining the beam distance on the light position detector 10 constant For this purpose, the light position detector 10 applies beam position signals 14a and 14b representing the beam positions to a servo control circuit 17. The latter 17 processes the beam position signals 14a and 14b to provide beam correcting signals 15a and 15b to move the movable reflectors 3a and 3b, thereby controlling the beam positions on the light position detector 10.
FIG. 10 (prior art) shows the arrangement of the light position detector 10 shown in FIG. 9 (prior art), and FIG. 11 (prior art) shows the arrangement of the servo control circuit 17 shown in FIG. 9 (prior art).
In the light position detector 10, as shown in FIG. 10 (prior art) the light beam 11a is applied to photo-detectors 20 and 21, which output beam position signals Va.sub.1 and Va.sub.2, respectively. Similarly, in response to the application of the light beam 11b to photo-detectors 22 and 23, beam positions signals Vb.sub.1 and Vb.sub.2 are output. As shown in FIG. 11 (prior art), the beam position signals Va.sub.1 and Va.sub.2 are applied to a difference output unit 24 which provides a difference signal 30. The difference signal 30 is Va.sub.1 -Va.sub.2. The difference signal 30 is amplified by an amplifier 26, the output of which is applied to a driver 28. In response to the output of the amplifier 26, the driver 28 applies the beam correcting signal 15a to the movable reflector 3a, so that the reflector 3a is so moved as to decrease the difference signal 30. As a result, the difference signal 30 is zeroed; i.e., Va.sub.1 -Va.sub.2 =0 is established with the light beam 11a positioned between the photo-detectors 20 and 21 Similarly, Vb.sub.1 -Vb.sub.2 =0 is established with the light beam 11b positioned between the photo-detectors 22 and 23. Thus, with the light beams 11a and 11b being servo-controlled, the beam distance (d') is maintained constant.
FIG. 12 (prior art) shows the arrangements of the light sources 1a and 1b which are semiconductor lasers, the light scan detector 8, and the control device 16. As shown in FIG. 12 (prior art), the light scan detector 8 comprises a front detector 31 and a rear detector 32, to which the beams 11a and 11b are applied successively, as a result of which the front detector 31 outputs a front passage signal 19a and the rear detector 32 outputs a rear passage signal 19b. Those passage signals 19a and 19b are applied to a difference output unit 33 which provides a difference analog output 34. The output 34 is supplied to a slicer 35 where it is sliced near OV. That is, the difference analog output 34 is sliced near the zero cross. The instants of time that the centers of the light beams 11a and 11b reach the boundary of the front detector 31 and the rear detector 32 are detected, and the difference in diameter of the light beams 11a and 11 b is compensated. Thus, the sliced signal is a pulse signal 37. The pulse signal 37 is applied to a timer 36 which is started by the trail edge 38 of the pulse signal 37 so that the timer 36 provides a timer output 39. The pulse signal 37 is further applied to AND gates 41. The timer output 39 is supplied to one of the AND gates 41 and an invertor 40, the output of said invertor 40 is applied to the other AND gate 41. Thus, the AND gates 41 output signals 43 and 44, respectively. These output signals 43 and 44 are converted by print timers 45a and 45b into print start signals indicating the arrival of the beams 11a and 11b to the print start positions respectively, to make access to line buffers 46a and 46b in which printing data has been stored, respectively. The line buffers 46a and 46b apply the printing signals to voltage-current exchangers 47a and 47b, which provide the modulating signals for the light source 1a and 1b, respectively. The outputs of the voltage-current exchangers 47a and 47b are applied to current adders 48a and 48b respectively.
The light sources 1a and 1b comprise: semiconductor lasers 49a and 49b; and laser power monitor sensors 50a and 50b, respectively. When energized, the semiconductor lasers 49a and 49b output the beams 11a and 11b and also output light beams to the laser power monitor sensors 50a and 50b in proportion to the beams 11a and 11b, respectively. In response to the light beams, the laser power monitor sensors 50a and 50b output beam power signals 13a and 13b which are supplied to power difference output units 51a and 51b, respectively. The power difference output unit 51a outputs the difference between the beam power signal 13a and a reference power voltage 52a and applies this output to a voltage-current exchanger 53a. The voltage-current exchanger 53a outputs current so that the difference between the beam power signals 13a and the reference voltage 52a is zeroed and applies a current signal to the current adder 48a. Similarly, the power difference output unit 51b outputs the difference between the beam power signal 13b and a reference power voltage 52b and applies this output to a monitor voltage-current exchanger 53b. The monitor voltage-current exchanger 53b outputs current so that the difference between the beam power signal 13b and the reference voltage 52b is zeroed and applies a current signal to the current adder 48b. The output current signals of the current adders 48a and 48b are applied to the semiconductor lasers 49a and 49b, so that the latter 49a and 49b provide the beams 11a and 11b with powers corresponding to the reference power voltages 52a and 52b, respectively. The two beams 11a and 11b can be maintained equal in power by adjusting the reference voltages 52a and 52b.
The above-described high-performance double beam scanning technique in which the positions of two beams are maintained constant, the difference between the beam diameters is compensated, and the beam powers are maintained unchanged suffers from the following possible reliability problems:
(1) The beams may become abnormal, therefore it is necessary to check the conditions of the two beams at all times.
(2) The cause of an abnormality is not always obvious, therefore when an abnormal condition occurs, it is necessary to detect what part of the apparatus is out of order.
Conventional techniques for solving the above-described problems (1) and (2) will be described.
A conventional technique which may solve the problem (1) has been disclosed by Japanese Patent Application (OPI) No. 67374/1982 (the term "OPI" as used herein means an "unexamined published application"). In the conventional technique, the front part of a photo-detector is designed so that the output signal of the photo-detector is sliced with a reference voltage, and the sliced signal is processed by count means and gate means so that a plurality of beams are distributed to line signals and the time interval between the first and last line signal is monitored with a timer. If this technique is applied to a double beam scanning technique, then the zero cross detection used for compensation of the difference between the beam diameters raises another problem. That is, since the slice point is near OV as shown in FIG. 12 (prior art), if the beam power is decreased due to an abnormal condition, the difference analog output 34 is still output even though it is low, and the pulse signal is output normally. Furthermore, as for the signal check, since the access signal of the count means is provided after the slicing of the output signal of the photo-detector, the spur due to the time delay of the count means is output through the gate means, thus lowering the control reliability. Also, the method of monitoring the first and last beam signals cannot handle a high speed beam scanning operation, thus the reliability is decreased. Therefore, it is necessary to provide a technique other than that disclosed by the Japanese Patent Application (OPI) No. 67374/1982.
With respect to the above-described problem (2), the different components employed in a double beam scan technique are each subject to different abnormalities. For instance, in FIG. 9 (prior art), the lens systems suffer from a problem of possible contamination, the beams 11a and 11b for some reason may not be applied to the movable reflectors 3a and 3b, and the light sources 1a and 1b are semiconductor lasers which have a specific service life and deteriorate over time. No technique for detecting these problems individually has been provided in the prior art.